The present invention relates to optical systems, and more specifically, to optical systems for scanning a beam of radiant energy across a moving workpiece to be acted upon by the radiant energy.
According to the present invention, there is provided an optical system comprising means for bringing a beam of radiant energy to a line focus upon a moving workpiece to be acted upon by the radiant energy, the arrangement being such that the line focus is perpendicular to the direction of movement of the workpiece, and means for scanning the focused beam across the workpiece in a direction having a component in the direction of motion of the workpiece such that the line focus of the beam of radiant energy traverses the workpiece in a straight line in the direction of the line focus.
The means for bringing the beam of radiation to a line focus may comprise a cylindrical focusing element, a spherical transfer focusing element, and a final spherical focusing element. The means for scanning the focused beam across the workpiece may comprise a mirror positioned between the transfer element and the final spherical element and mounted for motion about an axis the orientation of which can be varied to alter the direction of scan of the focused beam.
If it is desired to use a beam of radiation to act upon a workpiece, it is often beneficial if the beam of radiation is brought to a line focus of limited length, rather than a point focus. In order to achieve a narrow interaction zone between the beam of radiation and the workpiece it then becomes necessary to scan the line focused beam of radiation across the workpiece in a straight line which is a continuation of the focal line. No problems arise in doing this if the workpiece is stationary, but if the workpiece is moving and it is required that the line of interaction should be perpendicular to the length of the workpiece, then it is necessary to scan the line focus of the beam across the workpiece in a direction such that the line focus has a component of velocity equal to the velocity of the moving workpiece. It is also necessary for the line focus to move so that its direction remains parallel to its original direction.
Both these criteria can be satisfied by bringing the beam to a line focus upon a rotating polygonal mirror which is arranged to scan the reflected beam across a large aperture final focusing element which re-forms the line focus of the beam upon the workpiece, and arranging that an appropriate angle exists between the axis of rotation of the mirror and the direction of the original line focus. To allow for variations in the velocity of the workpiece there should be provided means for varying the orientation of the axis of rotation of the mirror as a function of the rate of scan of the beam of radiation across the workpiece and the velocity of the workpiece. Alternatively the mirror can be a plane mirror which is made to oscillate or rotate about an axis which lies in the plane of its reflecting surface.
According to a preferred embodiment of the invention there is provided an optical system for scanning a beam of radiant energy across a moving workpiece to be acted upon by the radiant energy, comprising a cylindrical mirror arranged to bring the beam of radiation to an initial line focus, a polygonal mirror mounted for rotation about an axis, the orientation of which can be varied, and arranged to scan the beam of radiation across a spherical mirror which is positioned to reform the image of the line focus of the beam of radiation on the workpiece, and means for varying the orientation of the axis of the polygonal mirror as a function of the rate of scan of the beam of radiation across the workpiece and the velocity of the workpiece so that the line-focused image is perpendicular to the direction of motion of the workpiece and traverses the workpiece in a straight line in the direction of the line focus.
The optical system may also include means for causing more than one line focus to be formed. For example, there may be included a split plane mirror or a Fresnel biprism, or when the cylindrical focusing element is a cylindrical mirror, as in the preferred embodiment, the cylindrical mirror can be divided symmetrically about a longitudinal axis and the two halves may be displaced angularly so that an angle exists between the normals to the two parts of the mirror along their line of contact. Three parallel line foci can be produced by incorporating into the system a three-facet plane mirror.
If the workpiece is a strip of plastics material such as high density polythene, and it is to be cut, or welded, of if it is in tubular form, cut and welded to form sealed lengths, then a suitable source for the beam of radiation is a laser which emits radiation in the infra-red region of the spectrum, such as that form of laser which uses carbon dioxide gas as the lasing medium.